Forests, like cities, can grow both by spreading out and by packing more things into the same space. Just as demographers do not tally the population of a city in square miles, so conservationists cannot get a complete picture knowing only the area of a forest—the usual measure of deforestation. Using data from the U.S. Forest Service and the United Nations, a team of American and Finnish researchers looked at changes in forest density, in addition to total area. The records covered 68 nations around the globe since 1990, and the United States, where detailed records go back further, since 1953.

Although forest area shrank by 10 percent in South America and Africa, it remained steady in the United States and expanded in Europe. Density increased everywhere but Asia, where rampant deforestation in Indonesia canceled out gains elsewhere on the continent. Overall, forests grew much denser, probably due to better forest management. Forests sequester carbon dioxide from the atmosphere, and more volume means more carbon stashed away.

The findings will help guide future policies intended to temper climate change, notes study author Iddo Wernick, a physicist at Rockefeller University. “If you want to offset a million tons of carbon dioxide,” he says, “how you do it depends on what you assume about the forests.”